Lightweight semi-conductor power supply for discharge lamps with means for utilizing inductive high voltage spikes to start the lamps



E. J. STURDEVANT ET LIGHTWEIGHT SEMI-CONDUCTOR POWER SUPPLY FOR DISCHARGE LAMPS Oct. 27, 1970 3,536,955

WITH MEANS FOR UTILIZING INDUCTIVE HIGH VOLTAGE SPIKES TO START THE LAMPS 2 Sheets-Sheet l Filed March 26, 1968 Y Oct. 27, 1970 E. J. STURDEVANT ETAL 3,536,955

LIGHTWEIGHT SEMI-CONDUCTOR POWER SUPPLY FOR DISCHARGE LAMPS WITH MEANS FOR UTILIZING INDUCTIVE HIGH VOLTAGE SPIKES TO START THE LAMPS v Filed March 2 6, 1968 '2 Sheets-Sheet 2 United States Patent LIGHTWEIGHT SEMI-CONDUCTOR POWER SUP- PLY FOR DISCHARGE LAMPS WITH MEANS FOR UTILIZING INDUCTIVE HIGH VOLTAGE SPIKES TO START THE LAMPS Eugene J. Sturdevant and Le Roy D. Phillips, Wilmington, Del., assignors to Holotron Corporation, Wilmington, Del., a corporation of Delaware Filed Mar. 26, 1968, Ser. No. 716,110 Int. Cl. Hb 37/00, 39/00, 41/14 US. Cl. 315-241 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates generally to electrical power supplies and more specifically to such power supplies designed for operating and starting gaseous discharge lamps.

Many types of gaseous discharge lamps, the most commonly used being of a mercury arc type, require a low voltage between their two operating electrodes relative to the usual 110 volt house power to operate the lamp once an arc has been established. To initially start such a lamp after it has remained unused for a period, a very high instaneous starting voltage must be established. Some types of lamps use two electrodes for both the operating and the starting functions and others use a starting electrode in addition to a pair of operating electrodes but the aforementioned voltage requirements apply to both types.

Existing gaseous discharge lamp power supplies generally take ordinary 60 cycle house power and transform to the two required lamp voltages by use of two independent electromagnetic transformers. Such transformers constitute a substantial weight and size of such a power supply.

It is, therefore, a primary object of this invention to provide the necessary voltages to operate a gaseous discharge lamp by a power supply that is substantially lighter in weight and smaller in physical size than existing power supplies.

It is a further object of this invention to provide a power supply for gaseous discharge lamps that is less costly to manufacture than those presently available.

It is a further object of this invention to provide a power supply with higher efiiciency than existing supplies.

SUMMARY OF THE INVENTION These and other objects of this invention are accomplished by a power supply with a circuit that converts the current of an existing power source (generally ordinary house power for convenience) to an alternating current of a very high frequency through the use of a pair of ICC switching transistors and associated circuitry and then transforms this alternating current to the value needed for sustained operation of the lamp. Because the alternating current transformed is of a high frequency compared to the normal 60 cycle house power, the transformer used requires considerably less core material with a resulting saving in size, weight, and cost of the entire supply. The instantaneous high starting voltage necessary to start a gaseous discharge lamp is obtained from a normally undesirable spiking voltage which occurs when current in the transformer is caused to rapidly change direction by a switching of the two transistors.

The invention, together with further objects and advantages thereof, may best be understood with reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an arc lamp power supply according to the preferred embodiment of the present invention.

FIG. 2 is a hologram viewer which utilizes the improved power supply of this invention.

FIG. 3 is a schematic view of an optical system within the hologram viewer of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT A typical watt direct current operating mercury lamp for use in portable equipment requires about 20 volts at approximately 5 amperes steady state operating current. Higher powered lamps require different running voltages and currents but all require some kind of power supply to convert the normal house power available into the required operating voltages and current for the lamp. In addition to this normal operating power, a high voltage is necessary for an instant to vaporize the mercury within the bulb when starting the lamp.

A preferred embodiment of the present invention is shown in the circuit schematic diagram of FIG. 1 where a mercury lamp 11 of the three terminal type is utilized with operating electrodes 13 and 15 and a starting electrode 17. For a typical 100 watt lamp, as referred to above, the 20 volt steady state power is required to be applied between the electrodes 13 and 15 (first and second terminals) and the instantaneous starting voltage required between the electrodes 15 and 17 (second and third terminals) is in excess of 3 kilovolts. The power supply circuit of FIG. 1 supplies these extreme voltage requirements from a power source of volt, 60 cycle house power. It should be understood, however, that the present invention is applicable with all its advantages to various characteristic power sources and different lamp power requirements.

For convenience in understanding the operation of the circuit of FIG. 1, it is divided into several blocks by dotted lines and will be explained first by reference to these blocks. Block 18 is a conventional rectifying circuit. The hot line of a 110 volt, 60 cycle alternating current power source is connected to a terminal 19, the neutral line of this supply connected to a terminal 21 and an external ground connected to a terminal 23. The output of block 18 will be approximately volts direct current between terminals 25 and 27, for the particular circuit shown.

A switching and transformation circuit 28 utilizes this direct current from the rectifier circuit 18 to generate an alternating current between terminals 29 and 31 of a voltage level to match the running requirements of the mercury lamp 11. Transistors Q1 and Q2 cooperate with a transformer T1 and other components to rapidly switch the polarity of voltage applied to a primary coil 33 of the transformer T1. This switching rate should be made so that the alternating current through the primary Winding 33 is greatly in excess of normal 60 cycle house power within a range consistent with a core material 35 of the transformer T1. This rapid alternating current then allows the transformer core 35 to be of a small size without comprising proper transformer action. For best results, the core 35 should be made from a high permeability ferromagnetic material having a substantially square hysteresis loop and which is light in weight and small in size. Transformation at a high frequency further reduces the coil sizes necessary for the required transformer action. The frequency of this alternating current can be controlled by controlling the number of wire turns of the primary coil 33, this frequency going up as the number of turns goes down and going down as the number of turns goes up. A secondary winding 37 of the transformer T1 is chosen to have a number of turns so that their ratio with the number of turns of the primary winding 33 is such to transform this alternating current to a voltage across the terminals 29 and 31 that is of the desired magnitude for proper operation of the mercury lamp 11.

A rectifying circuit 38 converts the alternating voltage between the terminals 29 and 31 into a direct current across terminals 39 and 41. The elements 13 and of the mercury lamp 11 are connected to this direct current source which provides the voltage and current of the required magnitudes for proper sustained operation of the lamp. It may be noted that the switching and transformation circuit 28 along with the rectifying circuit 38 function as a DC to DC converter.

Voltage across the primary winding 33 is switched rapidly from one direction to another to form a current wave shape in approximately a squarewaveform. This rapid switching causes very high voltage spikes at the instant of current switching. These voltage spikes are normally undesirable and much work has been done to eliminate their effect on other circuit components, especially upon the transistors Q1 and Q2. As part of the present invention, however, it was discovered that this voltage spike characteristic may be utilized to advantage to provide the instantaneous high voltage necessary to start the mercury lamp 11. A circuit with these voltage spikes present is connected to a high voltage circuit 42 at terminals 43 and 45. The high voltage spikes are shorted out through a normally closed switch, S2. However, when the circuit through the switch S2 is opened, these high voltage spikes will be increased in magnitude by an autotransformer T2 and presented for use at terminals 47 and 49. The starting element 17 of the mercury lamp 11 is connected to the terminal 47 and the running element 15 of the mercury lamp 11 is connected to terminal 49, thereby creating the necessary high voltage within the lamp to vaporize the mercury therein to start operation. It should be noted that the magnitude of these voltage spikes across the primary winding 33 is dependent upon how close its current waveform is to an ideal square wave, and this in turn depends upon how close the characteristics of the core material are to an ideal square hysteresis loop.

A more detailed description will now be presented of the circuit of FIG. 1, utilizing the present invention in a specific application to provide power to a mercury vapor lamp in a hand carried attache case wherein size and weight of the power supply circuit are very important. Normal house power applied at the terminals 19 and 21 is interrupted by a double pole, single throw switch S1,

which is the master power switch. A fan 51 is connected across the power line through a fuse 53 to provide movement of air for cooling the transistors Q1 and Q2 and the lamp 11, as well as other heat producing components confined in a small space. A fuse 55 is utilized for protection of the power supply circuit and a neon lamp 57 provides an indication of when power is being applied to the power supply circuit. The house power is applied across a current limiting power resistor R3 in series with a full-wave rectifier bridge circuit D3 made up of four semiconductor diodes. A full-wave rectified direct current then exists between terminals 25 and 27 with a filter capacitor C5 and a load resistor R4 in parallel therewith for smoothing out the ripples in the direct current.

The heart of the switching and transformation circuit 28 is the switching transistors Q1 and Q2 which cooperate with each other and other circuit components to produce a substantially square current waveform in the primary winding 33 of transformer T1. These transistors are preferably power transistors with high voltage ratings which then eliminate the need for a voltage reducing transformer in the rectifier circuit 18.

The transistor Q1 has its collector connected to one end of the center-tapped primary winding 33 and impresses a voltage across a winding portion 33:: when it is in a conducting state. The transistor Q2 has its collector connected to the opposite end of the centertapped primary winding 33 and impresses a voltage across a winding portion 33b when it is in a conducting state in a manner to produce magnetic flux in an opposite direction from that produced by the transistor Q1 and the winding portion 330. The center-tap connection of the winding 33 is connected to terminal 27, the positive side of the DC power supply 18, and is connected to the base of transistor Q1 through a resistor R2. The

emitters of the transistors Q1 and Q2 are joined and connected to terminal 25 which is the negative side of the DC power supply 18. The base elements of transistors Q1 and Q2 are connected to opposite sides of a feedback winding 59 of the transformer T1.

When the DC voltage across terminals 25 and 27 is initially applied to the circuit, a slight inherent unbalance which always exists in a circuit such as the switching and transformation circuit 28 will cause one of the transistors to begin conducting, resulting in a flux build-up in primary winding 33 which is induced in the feedback winding 59, driving the conducting transistor instantly to saturation. When a transistor is in saturation, the flux generated by the primary winding 33 collapses and induces a voltage in the feedback winding 59 of an opposite polarity which then switches the second transistor into its saturated state. This cyclical switching causes a square wave current in the primary coil 33 which is induced in the secondary coil 37 for rectification and application to the mercury lamp 11. Rectification is accomplished by a full-wave bridge rectifier D4 made up of four semiconductor diodes with its pulsating DC output being filtered by parallel filtering capacitor C4 and applied in series through a current-limiting resistor R5 to the mercury lamp 11. The resistor R5 is desirable for the initial operating period of the mercury lamp 11 since at that time the voltage between the elements 13 and 15 is extremely small.

A resistor R1 is provided in series with the feedback coil 59 to limit the base drive current applied to the transistors Q1 and Q2 from the voltage induced in this winding. Diodes D1 and D2 are connected between the base and emitter of the transistors Q1 and Q2, respectively, and function to provide a current path for the feedback Winding 59, to provide reverse bias for the transistors and to protect the base-emitter semiconductor junction of the non-conducting transistor from excessive reverse voltage.

The capacitors C1, C2, and C3, are connected between the collector and emitter of transistors Q2 and Q1 and connected between the bases of the two transistors, respectively. These are dc-spiking capacitors which function to eliminate the effect of the spiking voltages produced when the current in the primary winding 33 rapidly changes direction. These capacitors, in normal operation, bypass the high instantaneous voltages from the transistors Q1 and Q2, thus preventing their damage. One place in the circuit where the de-spiking voltage occurs, between the terminals 43 and 45, is utilized, as hereinabove described, to provide the high instantaneous starting voltage necessary to ignite the mercury lamp 11. In normal running operation of the lamp, switch S2 is closed, thus providing a current path through the capacitor C1 which protects the transistor Q1 from damage from the repetitive high voltage spikes. When the switch S2 is depressed, however, instead of bypassing this high voltage spike directly to ground, the autotransformer T2 having windings 46 and 48 is placed in the circuit to increase the magnitude of the voltage spikes to an instantaneous value of several thousand volts. The transformer T2 may be a very small lightweight device, thus presenting minimum size and weight for the components necessary to obtain this required high starting voltage. Since the transformer T2 has one side of the winding 46 connected to ground, the de-spiking circuit remains operable even when the switch S2 is depressed.

As a specific example of a circuit for operating a 100 watt mercury lamp with a 20 volt normal running voltage and requirement in excess of 3,000 volts to start the lamp, a set of component values which will result in a good power supply is tabulated below:

Q1 and Q2-400 volts, 100 watts, such as a Delco DTS 423 power transistor.

Tl-has a center-tapped primary winding 33 with 314 turns of N0. 70 AWG wire, a secondary coil 37 with 40 turns of No. 14 wire, and a feedback coil 59 with 7 turns of No. 23 AWG wire.

T2has winding 46 which functions as a primary winding and about 40 turns of No. 26 AWG wire; and a winding 48 which has a very high number of turns times that of winding 46, and has No. 38 AWG wire.

R1--39 ohms, 2 watts.

R210,000 ohms, watts.

R35 ohms, 50 watts.

R4--22,000 ohms, 50 watts.

R51 ohm, 50 watts.

C1-0.12 microfarad, 600 volts.

C20.12 microfarad, 600 volts.

C3O.12 microfarad, 600 volts.

C4150 microfarads, 150 volts.

C5-200 microfarads, 150 volts.

This particular circuit operates at a switching rate of approximately 3,000 cycles per second. For most lightweight ferromagnetic core materials commercially available, a frequency of operation from 1 to 3 kilocycles is compatible therewith.

Reference should be had to FIGS. 2 and 3 for an illustration of an application of the present invention. An attache case 61 contains the power supply circuit as illustrated in FIG. 1 along with a mercury lamp 11 and as sociated optical equipment to make a portable viewer for holograms. The mercury lamp 11 is surrounded by a reflecting and heat insulating shield 63 which allows a wedge of light 65 to fall upon a lens 67 for controlling the divergence of the light beam which is reflected in a mirror 69 onto a hologram held within a frame 71. A three-dimensional image may then be rendered from a hologram by looking through it toward the mirror 69. For holographic image reconstruction, a high intensity light source, such as produced by a mercury lamp is required and the power supply of the present invention for the mercury lamp allows such a viewer to be built into a norma lattache case and easily carried from one location to another to be operated from normal house power available at most locations.

While there has been described a preferred embodiment of the invention it will be understood that further modifications may be made without departing from the spirit and scope of the invention as set forth in the appended clairns. For instance, the power supply of this invention may be used with other loads with particular voltage requirements other than gaseous discharge lamps.

What is claimed is:

1. For an electrical load that requires continuous operating current and an occasional pulse of instantaneous high voltage independent of said operating current, an electrical power supply comprising,

a transformer including at least a primary winding to receive an alternating current and a secondary winding electrically connected to first and second power supply output terminals, said transformer additionally including a core of ferromagnetic material characterized by a substantially square hysteresis loop,

switching means connected to at least a portion of said primary winding for supplying thereto a voltage periodically and rapidly switched in magnitude in the form of a square-wave, thereby to produce an instantaneous high voltage pulse across said switching means each time the voltage is rapidly switched in magnitude,

means for suppressing said instantaneous high voltage pulses across said switching means, and

means connected to said suppressing means for directing at least one of said instantaneous high voltage pulses to a third and said second power supply output terminals when desired.

2. For a gaseous discharge lamp that requires continuous operating current for conduction through its contained gas and at least one instantaneous high voltage pulse for ionizing the contained gas to begin such conduction, an electrical power supply comprising,

a transformer including at least a primary winding to receive an alternating current and a secondary winding to supply said continuous operating current to said lamp, said transformer additionally including a core of ferromagnetic material characterized by a substantially square hysteresis loop,

switching means connected to at least a portion of said primary winding for supplying thereto a voltage periodically and rapidly switched in magnitude in the form of a square-wave having a frequency at least several times 60 cycles per second, whereby an instantaneous high voltage pulse is produced across said switching means each time the voltage is rapidly switched in magnitude,

means for suppressing said instantaneous high voltage pulses across said switching means, and

means connected to said suppressing means for directing at least one of said instantaneous high voltage pulses to said gaseous discharge lamp independently of said continuous operating current when desired to begin conduction of the continuous operating current through the contained gas of said lamp.

3. An electrical power supply according to claim 2 wherein said means for supplying a portion of the primary winding with a voltage periodically and rapidly switched in magnitude in the form of a square-wave includes a switching element connected in series with both at least a portion of said primary winding and a direct current power source, and additionally wherein said means for normally suppressing said instantaneous high voltage pulses includes a capacitor normally connected across said switching element.

4. An electrical power supply according to claim 2 wherein said means for suppressing the instantaneous high voltage pulses includes a capacitor connected across at least a portion of said switching means and wherein said means for directing at least one of said instantaneous References Cited UNITED STATES PATENTS 3,375,403 3/1968 Fliede 315240 3,289,098 11/1966 Cannalte 331-62 8 3,361,952 1/1968 Bishop 321-45 3,310,723 3/1967 Schmidt 3201 JOHN HUCKERT, Primary Examiner 5 B. ESTRIN, Assistant Examiner U.S. C1. X.R. 

